The largest muscle in the human body is the gluteus maximus. It is the uppermost of the three gluteal muscles that make up the human buttocks; the other two are gluteus medius and gluteus minimus. According to some definitions, it is the most powerful muscle of the human body. It is considered a part of the thigh muscle group. It extends thigh at the hip, and assists in laterally rotating the thigh.
During normal walking, hip extension is primarily a function of hamstrings...

The largest muscle in the human body is the gluteus maximus. It is the uppermost of the three gluteal muscles that make up the human buttocks; the other two are gluteus medius and gluteus minimus. According to some definitions, it is the most powerful muscle of the human body. It is considered a part of the thigh muscle group. It extends thigh at the hip, and assists in laterally rotating the thigh.
During normal walking, hip extension is primarily a function of hamstrings rather than gluteus maximus. When leg is fixed as in standing the gluteus maximus is an extensor of the pelvis on the thigh. When gluteus maximus is weak, the body lurches backward at heel-strike on weakened side to interrupt forward motion of the body.
Its large size is one of the most characteristic features of the muscular system in humans, connected as it is with the power he has of maintaining the body in the erect posture. The muscle is remarkably coarse in structure, being made up of fibres that are directed obliquely downward and lateral.

latissimus dorsi

I know the longest and largest muscle is the gluteus maximus (the butt).

the glutus maximus, maximus because it is the largest of the gluetus muscles

gluteus maximus